SIST ISO 22028-2:2014

INTERNATIONAL ISO
STANDARD 22028-2
First edition
2013-04-15
Photography and graphic
technology — Extended colour
encodings for digital image storage,
manipulation and interchange —
Part 2:
Reference output medium metric RGB
colour image encoding (ROMM RGB)
Photographie et technologie graphique — Codages par couleurs
étendues pour stockage, manipulation et échange d’image numérique —
Partie 2: Codage d’image en couleurs RVB par référence de sortie par
voie métrique
Reference number
ISO 22028-2:2013(E)
©
ISO 2013

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ISO 22028-2:2013(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2013
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Published in Switzerland
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ISO 22028-2:2013(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements . 5
4.1 General . 5
4.2 Reference viewing environment . 5
4.3 Reference medium . 6
4.4 ROMM RGB colour image encoding . 8
4.5 Inverse ROMM RGB transformation .10
Annex A (informative) Selection of ROMM RGB colour encoding .13
Annex B (informative) Conversion between ROMM RGB and video RGB .17
Bibliography .20
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ISO 22028-2:2013(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2. www.iso.org/directives
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received. www.iso.org/patents
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
The committee responsible for this document is ISO/TC 42, Photography.
This first edition cancels and replaces ISO/TS 22028-2:2006, which has been technically revised.
ISO 22028 consists of the following parts, under the general title Photography and graphic technology —
Extended colour encodings for digital image storage, manipulation and interchange:
— Part 1: Architecture and requirements
— Part 2: Reference output medium metric RGB colour image encoding (ROMM RGB)
— Part 3: Reference input medium metric RGB colour image encoding (RIMM RGB) [Technical Specification]
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ISO 22028-2:2013(E)

Introduction
This part of ISO 22028 has been developed in order to meet the industry need for a complete, fully-
documented, publicly-available definition of a wide-primary output-referred extended gamut red-
green-blue (RGB) colour image encoding. This colour image encoding provides a way to represent
output-referred images that does not limit the colour gamut to those colours capable of being displayed
on typical monitors, as is the case with the sRGB colour encoding, or require the use of negative RGB
colourimetry coordinates, as is the case with extended sRGB colour encodings like bg-sRGB.
An extended colour-gamut colour encoding is particularly desirable for professional photography
applications. For example, colours used for company logos can be outside a monitor gamut and would
therefore need to be clipped or compressed to a less saturated colour. Similarly, photographic prints
can contain colours outside a monitor RGB colour gamut. By using a standard output-referred extended
gamut colour image encoding, images containing such colours can be stored, interchanged, manipulated,
and later printed, without limiting or distorting the colours of the final output.
The Reference output medium metric RGB (ROMM RGB) colour image encoding specified in this part of
ISO 22028 meets the needs of these types of applications.
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INTERNATIONAL STANDARD ISO 22028-2:2013(E)
Photography and graphic technology — Extended colour
encodings for digital image storage, manipulation and
interchange —
Part 2:
Reference output medium metric RGB colour image
encoding (ROMM RGB)
IMPORTANT — The electronic file of this document contains colours which are considered to be
useful for the correct understanding of the document. Users should therefore consider printing
this document using a colour printer.
1 Scope
This part of ISO 22028 defines a family of extended colour-gamut output-referred RGB colour image
encodings designated as reference output medium metric RGB (ROMM RGB). Digital images encoded
using ROMM RGB can be manipulated, stored, transmitted, displayed, or printed by digital still picture
imaging systems. Three precision levels are defined using 8-, 12- and 16-bits/channel.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 15076-1:2010, Image technology colour management — Architecture, profile format and data
structure — Part 1: Based on ICC.1:2010
ISO 22028-1:2004, Photography and graphic technology — Extended colour encodings for digital image
storage, manipulation and interchange — Part 1: Architecture and requirements
ISO 11664-1:2007, (CIE S 014-1/E:2006) Colorimetry – Part 1: CIE standard colorimetric observers
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
adapted white
colour stimulus that an observer who is adapted to the viewing environment would judge to be perfectly
achromatic and to have a luminance factor of unity; i.e. absolute colorimetric coordinates that an
observer would consider to be a perfect white diffuser
Note 1 to entry: The adapted white can vary within a scene.
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3.2
additive RGB colour space
colorimetric colour space having three colour primaries (generally red, green and blue) such that CIE
XYZ tristimulus values can be determined from the RGB colour space values by forming a weighted
combination of the CIE XYZ tristimulus values for the individual colour primaries, where the weights are
proportional to the radiometrically linear colour space values for the corresponding colour primaries
Note 1 to entry: A simple linear 3 × 3 matrix transformation can be used to transform between CIE XYZ tristimulus
values and the radiometrically linear colour space values for an additive RGB colour space.
Note 2 to entry: Additive RGB colour spaces are defined by specifying the CIE chromaticity values for a set of
additive RGB primaries and a colour space white point, together with a colour component transfer function.
3.3
colorimetric colour space
colour space having an exact and simple relationship to CIE colorimetric values
Note 1 to entry: Colourimetric colour spaces include those defined by CIE (e.g. CIE XYZ, CIELAB, CIELUV), as well
as colour spaces that are simple transformations of those colour spaces (e.g. additive RGB colour spaces).
3.4
colour component transfer function
single variable, monotonic mathematical function applied individually to one or more colour channels
of a colour space
Note 1 to entry: Colour component transfer functions are frequently used to account for the nonlinear response
of a reference device and/or to improve the visual uniformity of a colour space.
Note 2 to entry: Generally, colour component transfer functions will be nonlinear functions such as a power-law
(i.e. “gamma”) function or a logarithmic function. However, in some cases a linear colour component transfer
function can be used.
3.5
colour encoding
generic term for a quantized digital encoding of a colour space, encompassing both colour space
encodings and colour image encodings
3.6
colour gamut
solid in a colour space, consisting of all those colours that are either: present in a specific scene, artwork,
photograph, photomechanical, or other reproduction; or capable of being created using a particular
output device and/or medium
3.7
colour image encoding
digital encoding of the colour values for a digital image, including the specification of a colour space
encoding, together with any information necessary to properly interpret the colour values such as the
image state, the intended image viewing environment and the reference medium
Note 1 to entry: In some cases, the intended image viewing environment will be explicitly defined for the colour
image encoding. In other cases, the intended image viewing environment can be specified on an image-by-image
basis using metadata associated with the digital image.
Note 2 to entry: Some colour image encodings will indicate particular reference medium characteristics, such as
a reflection print with a specified density range. In other cases, the reference medium will be not applicable, such
as with a scene-referred colour image encoding, or will be specified using image metadata.
Note 3 to entry: Colour image encodings are not limited to pictorial digital images that originate from an original
scene, but are also applicable to digital images with content such as text, line art, vector graphics and other forms
of original artwork.
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ISO 22028-2:2013(E)

3.8
colour rendering
mapping of image data representing the colour space coordinates of the elements of a scene to output-
referred image data representing the colour space coordinates of the elements of a reproduction
Note 1 to entry: Colour rendering generally consists of one or more of the following:
— compensating for differences in the input and output viewing conditions;
— tone scale and gamut mapping to map the scene colours onto the dynamic range and colour gamut of the
reproduction;
— applying preference adjustments.
3.9
colour space
geometric representation of colours in space, usually of three dimensions
[CIE Publication 17.4:1987, 845-03-25]
3.10
colour space encoding
digital encoding of a colour space, including the specification of a digital encoding method, and a colour
space value range
Note 1 to entry: Multiple colour space encodings can be defined based on a single colour space where the different
colour space encodings have different digital encoding methods and/or colour space value ranges. (For example,
8-bit sRGB and 10-bit e-sRGB are different colour space encodings based on a particular RGB colour space.)
3.11
colour space white point
colour stimulus to which colour space values are normalized
Note 1 to entry: It is not necessary that the colour space white point correspond to the assumed adapted white
point and/or the reference medium white point for a colour image encoding.
3.12
continuous colour space values
real-valued, unbounded colour space values that have not been encoded using a digital encoding method
3.13
extended gamut
colour gamut extending outside that of the standard sRGB reference display as defined by IEC 61966-2-1
3.14
gamut mapping
mapping of the colour space coordinates of the elements of a source image to colour space coordinates
of the elements of a reproduction to compensate for differences in the source and output medium colour
gamut capability
Note 1 to entry: The term “gamut mapping” is somewhat more restrictive than the term “colour rendering”
because gamut mapping is performed on colourimetry that has already been adjusted to compensate for viewing
condition differences and viewer preferences, although these processing operations are frequently combined in
reproduction and preferred reproduction models.
3.15
ICC profile
International Color Consortium’s file format, used to store transforms from one colour encoding to another,
e.g. from device colour coordinates to profile connection space, as part of a colour management system
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3.16
image state
attribute of a colour image encoding indicating the rendering state of the image data
Note 1 to entry: The primary image states defined in this document are the scene-referred image state, the
original-referred image state and the output-referred image state.
3.17
luminance factor
ratio of the luminance of the surface element in the given direction to that of a perfect reflecting or
transmitting diffuser identically illuminated
[CIE Publication 17.4:1987, 845-04-69]
3.18
medium black point
neutral colour with the lowest luminance that can be produced by an imaging medium in normal use,
measured using the specified measurement geometry
Note 1 to entry: It is generally desirable to specify a medium black point that has the same chromaticity as the
medium white point.
3.19
medium white point
neutral colour with the highest luminance that can be produced by an imaging medium in normal use,
measured using the specified measurement geometry
3.20
output-referred image state
image state associated with image data that represents the colour space coordinates of the elements of
an image that has undergone colour rendering appropriate for a specified real or virtual output device
and viewing conditions
Note 1 to entry: When the phrase “output-referred” is used as a qualifier to an object, it implies that the object
is in an output-referred image state. For example, output-referred image data are image data in an output-
referred image state.
Note 2 to entry: Output referred image data are referred to the specified output device and viewing conditions. A
single scene can be colour rendered to a variety of output-referred representations depending on the anticipated
output viewing conditions, media limitations, and/or artistic intents.
Note 3 to entry: Output-referred image data can become the starting point for a subsequent reproduction process.
For example, sRGB output-referred image data are frequently considered to be the starting point for the colour
re-rendering performed by a printer designed to receive sRGB image data.
3.21
tristimulus values
amounts of the three reference colour stimuli, in a given trichromatic system, required to match the
colour of the stimulus considered
[CIE Publication 17.4:1987, 845-03-22]
3.22
veiling glare
light, reflected from an imaging medium, that has not been modulated by the means used to produce
the image
Note 1 to entry: Veiling glare lightens and reduces the contrast of the darker parts of an image.
Note 2 to entry: In CIE Publication 122, the veiling glare of a display is referred to as ambient flare.
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ISO 22028-2:2013(E)

3.23
viewing flare
veiling glare that is observed in a viewing environment but not accounted for in radiometric measurements
made using a prescribed measurement geometry
Note 1 to entry: The viewing flare is expressed as a percentage of the luminance of adapted white.
4 Requirements
4.1 General
Reference output medium metric RGB (ROMM RGB) is an extended gamut RGB colour image encoding
for representing the colourimetry of output-referred image data in an output-referred image state on a
reference medium. The output-referred image data has the intended colour appearance when viewed in
a reference viewing environment. The image colourimetry is encoded in terms of an additive RGB colour
space associated with a hypothetical additive colour device having a specified set of primaries, no cross-
talk between the colour channels and a luminance dynamic range defined by an associated medium
black point and medium white point.
Three different precision levels are defined, and shall be identified as ROMM8 RGB, ROMM12 RGB and
ROMM16 RGB, for 8-, 12- and 16-bits/channel (24-, 36- and 48-bits/pixel) representations, respectively.
The image colourimetry shall be based on flareless (or instrument flare corrected) colourimetric
measurements. All chromaticity and tristimulus values specified in this document shall be based on the
CIE 1931 two-degree Standard Observer defined in ISO 11664.
Flareless colourimetric measurements should be considered equivalent to those obtained from real
reflection media measured using the 0/45 geometry specified in ISO 13655 (without polarizing means).
Therefore, colourimetric quantities referred to in this part of ISO 22028 should be considered to include
a level of flare typical of such measurements.
The colour image encoding defined in this International Standard conforms to the requirements defined
in ISO 22028-1:2004, Clause 5.
4.2 Reference viewing environment
The reference viewing environment shall be such that the adapted white has the chromaticity values of
CIE Illuminant D (x = 0,345 7, y = 0,358 5).
50 0 0
2
The absolute luminance level of the adapted white in the reference viewing environment shall be 160 cd/m .
NOTE 1 This absolute luminance level is equivalent to that of a perfect white Lambertian reflector illuminated
with 500 lx as specified in ISO 3664 for the practical appraisal of prints.
NOTE 2 The luminance of the adapting field can be assumed to be 20 % of the luminance of the adapted white.
The reference viewing environment shall be characterized by an “average” surround. This means that
the area immediately surrounding the image border shall be assumed to be a uniform grey having the
chromaticity values of CIE Illuminant D (x = 0,345 7, y = 0,358 5) and a luminance factor of 0,2
50 0 0
relative to the adapted white.
The reference viewing environment shall be assumed to have a level of viewing flare that is 0,75 % of the
adapted white with the chromaticity values of CIE Illuminant D (x = 0,345 7, y = 0,358 5).
50 0 0
NOTE 3 If the actual output viewing environment differs significantly from that specified here, appropriate
transformations might be necessary to determine the corresponding colourimetry that would produce the
intended colour appearance in the reference viewing environment. However, for actual viewing environments
similar to the reference viewing environment, it might not be necessary to make such adjustments. The reference
viewing environment was selected to make such adjustments unnecessary for many practical applications.
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ISO 22028-2:2013(E)

4.3 Reference medium
4.3.1 Reference medium white point
ROMM RGB shall be an encoding of the colours of an image on a reference medium having a reference
medium white point with the chromaticity values of CIE Illuminant D (x = 0,345 7, y = 0,358 5) and
50 0 0
a luminance factor of F = 0,89 relative to the adapted white. Accordingly, the reference medium white
W
point tristimulus values are X = F X = 85,81, Y = F Y = 89,00 and Z = F Z = 73,42, where
W W 0 W W 0 W W 0
X = 96,42, Y = 100,00 and Z = 82,49 are the tristimulus values of the adapted white.
0 0 0
NOTE 1 The luminance factor of 0,89 corresponds to a visual density of 0,050 6.
2
NOTE 2 The luminance factor of 0,89 corresponds to a reference medium white point luminance of 142 cd/m .
4.3.2 Reference medium black point
The reference medium shall have a reference medium black point with the chromaticity values of CIE
Illuminant D (x = 0,345 7, y = 0,358 5) and a luminance factor of F = 0,003 091 1 relative to the
50 0 0 K
adapted white. Accordingly, the reference medium black point tristimulus values are X = F X = 0,298 0,
K K 0
Y = F Y = 0,309 1 and Z = F Z = 0,255 0.
K K 0 K K 0
NOTE 1 The luminance factor of 0,003 091 1 corresponds to a visual density of 2,509 9.
NOTE 2 The luminance factor of 0,003 091 1 corresponds to a reference medium black point luminance of
2
0,495 cd/m .
4.3.3 Reference medium primaries
The x-y chromaticity values for the ROMM RGB primaries shall be as given in Table 1. Rationale for the
choice of these primaries is given in Annex A.
The colour space white point, corresponding to equal amounts of the three RGB primaries, shall have the
x-y chromaticity values of CIE Illuminant D given as given in Table 1.
50
Table 1 — CIE chromaticities for reference medium primaries and white point
CIE chromaticities
Reference medium primaries
and white point
a a
x y u’ v’
Red 0,734 7 0,265 3 0,623 4 0,506 5
Green 0,159 6 0,840 4 0,050 0 0,592 5
Blue 0,036 6 0,000 1 0,050 0 0,000 3
White point 0,345 7 0,358 5 0,209 2 0,488 1
a
  The u’-v’ chromaticity values for the RGB primaries and colour space white point given in this table can be derived from
the x-y chromaticity values and are provided for information purposes.
4.3.4 Reference medium rendering intent
ROMM RGB is an encoding of the intended colour appearance of an output-referred image rendered to
the reference output medium dynamic range and viewing environment. Images encoded in ROMM RGB
may contain colours that are outside the colour gamut for an actual output medium. Therefore, in order
to display an image encoded in ROMM RGB on an actual output medium, the colour values need to be
mapped to those that can be produced on that medium. All such mappings should be designed to retain
the encoded colour appearance as closely as possible. If the actual viewing environment is significantly
different from the reference viewing environment, the colour mapping from ROMM RGB to the actual
output medium should include appropriate colour appearance transformations to determine the
corresponding colourimetry that will maintain the colour appearance of the ROMM RGB colour values
in the reference viewing environment.
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ISO 22028-2:2013(E)

Image colourimetry encoded as ROMM RGB shall not contain colours outside the spectrum locus.
Figure 1 shows that the ROMM RGB encoding gamut extends outside the spectral locus between 465 and
485, and between 530 and 570 nm. As both the spectrum locus and the ROMM RGB encoding boundary
are constant in chromaticity, only the chromaticity needs to be considered when determining whether
a particular ROMM RGB triplet is allowed.
y
0,8
0,6
0,4
0,2
0,1 0,2 0,3 0,4 0,5 0,6 0,7 x
Figure 1 — x,y chromaticity plot of the spectrum locus and the ROMM RGB encoding gamut
Consideration should be given to the gamut limitations of real media in the process of performing colour
rendering to produce image colourimetry for encoding as ROMM RGB. For example, if the encoded image
colourimetry contains many colours which are far outside of the colour gamut for most real output
media, the colour mapping from ROMM RGB to an actual output media will be forced to significantly
distort these colours, which can lead to inconsistent results. The perceptual reference medium gamut
specified in ISO 15076-1 provides a reasonable target colour gamut for colour rendering to produce
image colourimetry for encoding as ROMM RGB. Figure 2 provides a view of the ROMM RGB encoding
gamut and the ISO 15076-1 perceptual reference medium gamut.
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ISO 22028-2:2013(E)

Figure 2 — Media-relative CIELAB plot of the ISO 15076-1 perceptual reference medium gamut
(black wireframe) and the ROMM RGB encoding gamut (transparent gray)
NOTE In some cases, the colour appearance encoded in ROMM RGB might not be ideally suited for display
on a particular real output media in a particular viewing environment. For example, the visual appearance of a
high-quality transparency, brightly illuminated with a dark surround, will typically be different from that of a
high-quality reflection print of the same scene viewed using a moderate illumination level. Therefore, in some
applications, it can be desirable to alter the colour appearance description provided by the ROMM RGB encoding
accordingly to produce the optimal image for the intended output media and viewing environment.
4.4 ROMM RGB colour image encoding
4.4.1 Encoding principles
ROMM RGB colour image encoding values shall be determined from tristimulus values of an image
on the reference medium using a matrix transformation (see 4.4.3) followed by a colour component
transfer function (see 4.4.4) and a digital encoding for one of three different bit-depths (see 4.4.5). The
image tristimulus values shall be those that produce the intended colour appearance when viewed in
the reference viewing environment.
NOTE Images intended to be viewed in other viewing environments, or on a medium different from the
reference medium, can be encoded in ROMM RGB by first determining the corresponding tristimulus values that
are expected to produce the intended colour appearance on the reference medium when viewed in the reference
viewing environment. The corresponding tristimulus values can be determined by using an appropriate colour
appearance transformation to account for the differences between the viewing conditions. Additionally, it might
be necessary to account for differences in the media characteristics.
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4.4.2 Tristimulus value normalization
The image tristimulus values shall be normalized such that the normalized Y tristimulus value of the
reference medium white point is 1,0 and the normalized Y tristimulus value of the reference medium
black point is 0,0.
()XX− X
K W
X =
N
()XX− Y
WK W
()YY−
K
Y =
N
()YY−
WK
()ZZ− Z
K W
Z =
N
()ZZ− Y
WK W
(1)
where X, Y and Z are the image tristimulus values; X , Y and Z are the normalized image tristimulus
N N N
values; X , Y and Z are the tristimulus value of the reference medium white point given i
...